slight miss at high speed

[dsujen]

I have a 1987 j90tlcur on a 16' aluminum boat. Tested motor on boat yesterday. Ran but miss fired. Came back to dock pulled spark plugs. Top right cylinder was dusty black. Other three were good. Installed new plugs and put on two new plug wires. That's all I had with me. Seemed to run better but at high speed it would buck slightly like it wanted to go faster. I'm not sure if it is the prop slipping or a bad coil or wires. I have 2 different types of wires on it now. At home on muffs at night after the test run, I pulled the new spark plugs and the top two cylinders were black and a little oily. The others looked decent. I sprayed the motor at night with water while running and saw no arcing at the wires. I put on a brand new old style fuel pump, brand new thermostats, and brand new fuel tanks before the test run. What can I do ?

 

[emdsapmgr]

A compression test would be a good place to start. That's a vital sign for good performance. Also, if the ignition system is losing spark once the engine warms up, you can do some testing. Check spark either with the cyl-drop test or with an inductive timing light. If you find spark is missing on one cyl, swap the two identical power packs and see if the "no spark" condition moves to the other head. That's a sign of a bad pack. Let us know what you find out.

 

[dsujen]

I did a compression check with the motor cold. It is top left 135 bottom left 125 top right 115 bottom right 110. The motor was sitting for an unknown time. I bought it non running with the vro set up. I changed that to a traditional pump and mix 50:1. I have portable tanks on board. Is it okay to run the portable tanks with the lids loose because that's how I ran it. The new tanks swell up like balloons.The prop on the motor is 13 3/4 x 15. I have no tach but I know the prop is too small because it takes off like a rocket and winds up too high. Top speed is 44 mph.

 

[racerone]

Compression values are not promising.---Run with timing light hooked up to see if there is a miss on one particular cylinder.--------I would remove the bypass cover on the cylinder with lowest compression. Allows you to have a look at piston and rings.------Just 6 small screws and you may need a new $1.00 gasket.

 

[dsujen]

The compression numbers are are acceptable according to the service manual. The motor doesn't stumble or miss fire at all when accelerated. It feels like it is starving for gas. I did notice that when I push the key in for cold starting that some gas spills out of the choke injector near the red handle. Could this be causing the problem? Is that thing fixable?

 

[emdsapmgr]

Usually, the compression numbers need to be within 10% of each other. Yours is outside that normal range, so you may want to go looking for a problem to ease your mind. Perhaps someone changed the port head to an older (high compression) head. That could explain the compression difference. Most of the factory heads of that year did not make 110 lbs compression. Older heads ran up to 125 or 135, depending on head part number. Any fuel tank needs to be vented when running. The red choke handle is replaceable. Bombardier sells it with orings. The replacement has been upgraded to include the Schrader valve-a nice feature. P/N 175158 Any results on the spark test?